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Chapter 9: Virtual Memory. Chapter 9: Virtual Memory. Background Demand Paging Copy-on-Write Page Replacement Allocation of Frames Thrashing. Objectives. To describe the benefits of a virtual memory system
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Chapter 9: Virtual Memory Background Demand Paging Copy-on-Write Page Replacement Allocation of Frames Thrashing
Objectives To describe the benefits of a virtual memory system To explain the concepts of demand paging, page-replacement algorithms, and allocation of page frames To discuss the principle of the working-set model
Background Chapter 8 discussed separation of logical memory from physical memory Used page tables and TLBs for address translation Assumption – Entire address space should be in main memory before process can begin execution Dynamic loading addressed this. But needed user to manage the loading process Virtual memory takes this separation to its logical conclusion Allows a program to be executed even when part of process’s address space is in main memory
The Case for Virtual Memory Programs have code for error conditions that are seldom used Programs have features that are seldom used Programmers allocate way too much memory than needed Create a 100X100 array when only 10X10 is needed Advantages of virtual memory Program no longer constrained by physical memory size More processes can be in physical memory thereby increasing system performance Less I/O for swapping Makes programmers task easier
Virtual Address Space Virtual address space – Logical view of how process is stored in main memory The virtual address space is again contiguous (recall logical address space is always contiguous) Physical memory organized as frames Holes in virtual address spaces – the blank space between stack and heap Physical memory allocated only if heap or stack grows Virtual address space with holes referred to as sparse spaces
Advantages of Virtual Address Space Efficient process creation (less space is allocated at the beginning) System libraries can be shared by mapping the libraries to virtual address space Virtual memory also helps in sharing memory space between processes Shared memory is a mechanism for cooperating processes to communicate Allows memory sharing when forking – efficient process creation
Demand Paging Bring a page into memory only when it is needed Less I/O needed Less memory needed Faster response More users Page is needed reference to it invalid reference abort not-in-memory bring to memory Lazy swapper– never swaps a page into memory unless page will be needed Swapper that deals with pages is a pager
Demand Paging Virtual memory can be implemented via Demand Paging or Demand Segmentation Bring a page into memory only when it is needed Till then it resides in secondary memory Pages that are never used (such as error handling code) never brought into main memory Similar to swapping Swapping operates at granularity of entire address spaces On-demand paging operates at granularity of pages/frames Lazy swapper – never swaps a page until it is needed Swapper that operates at granularity of pages is called Pager
Basic Concepts At process swap-in time pager makes informed guess on which pages will be used before process is swapped out Only those pages are brought into main memory More efficient– reduces process swap time Other pages are obtained on demand We need a mechanism to check whether page is in main memory or on disk Done with hardware support Valid and invalid bits
Valid-Invalid Bit With each page table entry a valid–invalid bit is associated(v page is legal and in-memory,i page is illegal or not in-memory) During address translation, if valid–invalid bit in page table entry is Ipage fault occurs Frame # valid-invalid bit v v v v i i i page table ….
How to Handle Page Fault Page fault traps into OS 1. Operating system looks at another table to decide: - Invalid reference abort - Just not in memory 2. Get empty frame 3. Swap page into frame 4. Reset tables 5. Set validation bit = v 6. Restart the instruction that caused the page fault
Performance of Demand Paging Page Fault Rate 0 p 1.0 if p = 0 no page faults if p = 1, every reference is a fault Effective Access Time (EAT) EAT = (1 – p) x memory access + p (page fault overhead + swap page out + swap page in + restart overhead)
Demand Paging Example Memory access time = 200 nanoseconds Average page-fault service time = 8 milliseconds EAT = (1 – p) x 200 + p (8 milliseconds) = (1 – p) x 200 + p x 8,000,000 = 200 + p x 7,999,800 If one access out of 1,000 causes a page fault, then EAT = 8.2 microseconds. This is a slowdown by a factor of 40!!
Process Creation Virtual memory allows other benefits during process creation: - Copy-on-Write - Memory-Mapped Files (later)
Copy-on-Write Mechanism for improving efficiency of process creation via fork() Copy-on-Write (COW) allows both parent and child processes to initially share the same pages in memory If either process modifies a shared page, only then is the page copied COW allows more efficient process creation as only modified pages are copied Free pages are allocated from a pool of zeroed-out pages Vfork() – Does not use COW. Changes made by child visible to parent
What happens on Page Fault? Find the location of the desired page on disk Find a free frame: - If there is a free frame, use it - If there is no free frame, use a page replacement algorithm to select a victimframe Bring the desired page into the (newly) free frame; update the page and frame tables Restart the process
What happens if there is no free frame? Find some page in memory, but not really in use and swap it out The page that is selected for swap-out is called the victim Performance optimizations Avoid two page transfer costs by having a dirty bit Only dirty pages are written back Reducing the need for swap-in/swap-out Reduce the page-fault rate
When Does Page Faults Occur? First time a frame is used – Mandatory page fault Little opportunity for optimization Page fault on subsequent accesses Page was swapped out between accesses to make room for an incoming page Having smart victim selection schemes can help reduce these faults Page replacement policy defines how the victim is selected Completes the separation between logical and physical memories
Page Replacement Algorithms Goal Minimize page-fault rate How to evaluate page replacement algorithms Simulation using memory access trace files Memory addressed can be translated to page numbers – page access string In all our examples, the reference string is 7, 0, 1, 2, 0, 3, 0, 4, 2, 3, 0, 3, 2, 1, 2, 0, 1, 7, 0, 1
FIFO Page Replacement • Replace the page that was swapped-in the earliest
FIFO Easy to implement Poor performance – high page fault rate FIFO also suffers from another problem called Belady’s anamoly Reference string: 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5 Page faults when # of frames is 3: 9 Page faults when # of frames is 4: 10 Belady’s Anomaly: More frames can actually result in higher page faults for certain access patters
Optimal Algorithm Replace page that will not be used for longest period of time 4 frames example Provably optimal 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5 Problem: How to know which page is not going to be used for longest time? Used for as a baseline for measuring performance of other algorithms 1 4 2 6 page faults 3 4 5
Least Recently Used Algorithm Replaces the frame that has not been used for the longest time Based on the assumption that page that has not been used for longest time will not be used in the near future Does not suffer from Belady’s anomaly
LRU Algorithm Implementation Counter implementation: Keep track of time the page was last used (e.g., have additional field in page table) Requires searching for finding the victim Stack implementation – keep a stack of page numbers in a double link form: Page referenced: move it to the top requires 6 pointers to be changed No search for replacement
LRU Approximation Algorithms Use reference bit associated with page table One-Reference bit With each page associate a bit, initially = 0 When page is referenced bit set to 1 Replace the one which is 0 (if one exists) We do not know the order, however Additional reference bit algorithm Keep a byte for ever entry in page table Move the bit from page table to MSB of the corresponding byte Replace the page with smallest integer (there could be many)
Second-Chance Algorithm Special case of Additional-reference bit algorithm Uses the reference bit in the page table Clock replacement – Maintain page references in a circular queue Pointer or clock handle always points to the next page that needs to be considered for replacement On page fault check the reference bit of the page If reference bit is zero, replace the page If reference bit is 1 then: Set reference bit 0 Do not replace Move to the next page and apply the same rules
Counting Algorithms Keep a counter of the number of references that have been made to each page LFU Algorithm: replaces page with smallest count MFU Algorithm: based on the argument that the page with the smallest count was probably just brought in and has yet to be used
Allocation of Frames How many frames to allocate to each process? Conditions: Total # of allocated frames over all processes < # of available frames Each process needs minimum number of pages Recall that instruction that caused page fault will have to be restarted Avoid multiple restarts of instruction Determined by instruction set Number of memory location than can be accessed in the instruction Level of indirections allowed
Equal Allocation All processes get the same number of frames Easy to implement Poor performance – One process could be wasting memory when another is short on memory
Proportional Allocation Proportional allocation – Allocate according to the size of process Better performance
Global vs. Local Allocation Page replacement has an impact on page allocation Global replacement– process selects a replacement frame from the set of all frames; one process can take a frame from another # of frames allocated to a process can change over time Memory reference pattern of one process can affect performance of another process Local replacement– each process selects from only its own set of allocated frames # of frames allocated to a process remains constant Restricts opportunities for dynamic optimization
Thrashing If a process does not have sufficient number of frames it constantly page faults High page fault rates leads to low performance High paging activity is called Thrashing A process that is thrashing spends more time paging than actual execution
Self Perpetuating Phenomenon In early batch systems, if CPU utilization is too low OS increases degree of multi-programming Global replacement is used – replaces page without regard to the process to which it belongs One processes needs more frames It faults and takes sway frame from other processes These other processes see increase faulting and queue up at the pager CPU scheduler sees lower CPU utilization and further increases degree of multi-programming
